The present invention relates to an ion guide or ion trap, a mass spectrometer, a method of guiding ions and a method of mass spectrometry.
Stacked ring ion guides are well known and comprise a plurality of ring electrodes each having an aperture through which ions are transmitted. The ion confining region of conventional stacked ring ion guides is circular in cross section.
It is known to increase the capacity of a conventional stacked ring ion guide by increasing the radius of the aperture to allow ions to occupy a larger volume. However, it becomes progressively harder to apply a transient DC or travelling DC voltage wave to such a device in order to urge ions along the length of the on guide due to the fact that the electric field relaxes within the ion guide. The relaxation of the electric field weakens the electric field experienced by ions towards the centre of the ion confining region for a fixed applied transient DC voltage. A higher transient DC voltage is therefore required in order to propel ions through the device. However, this can become problematic.
If it is required to apply a linear or non linear DC electric field over the axial length of the ion guide, then electric field penetration or relaxation at the entrance and exit of the device can cause significant disruption of the electric field.
Stacked ring ion guides are also known which have elliptical or rectangular apertures. Such ion guides effectively stretch the ion guide region in one radial direction without increasing the size of the aperture in the other radial direction. However, such ion guides suffer from the problem that electric field effects at the extremes of the device in the radial direction of elongation prevent ions from occupying the entire internal volume.
Cylindrical Field Asymmetric Ion Mobility Spectrometry (“FAIMS”) devices are also known and are conventionally operated at atmospheric pressure. A FAIMS device may comprise an inner cylindrical electrode and an outer cylindrical electrode. An asymmetric DC voltage waveform is applied between the inner and outer electrodes at atmospheric pressure resulting in some focussing for ions which have a specific difference in ion mobility in a high electric field compared to a low electric field. It will be understood by those skilled in the art that ions are not confined radially within the FAIMS device in either radial direction by a RF or pseudo-potential barrier.
The limited space charge capacity of conventional ion traps and ion guides can result ire loss of transmission or sensitivity due to inefficient ion confinement which leads to ion losses. Furthermore, conventional ion traps and ion guides may suffer from loss of analytical performance when used as an ion mobility separator (“IMS”) or mass to charge ratio separator. This is characterised by loss of resolution or separation power and/or by unexpected shifts in on ejection times. These shifts lead to inaccuracy of analytical measurements.
It is therefore desired to provide an improved ion guide.